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Pre-Main Sequence Accretion Rates at Low Metallicity 3 Draft version December 5, 2014 Preprint typeset using LATEX style emulateapj v. 05/12/14 PRE-MAIN SEQUENCE ACCRETION IN THE LOW METALLICITY GALACTIC STAR-FORMING REGION Sh 2-284 V. M. Kalari & J. S. Vink Armagh Observatory, College Hill, Armagh, BT61 9DG, UK; [email protected] Draft version December 5, 2014 ABSTRACT We present optical spectra of pre-main sequence (PMS) candidates around the Hα region taken with the Southern African Large Telescope, SALT, in the low metallicity (Z) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of Z 1/5 Z . It has been suggested on the basis of both theory and observations that PMS mass-accretion∼ rates, M˙ acc, are a function of Z. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-Z star-forming region. Our data-set was enlarged with literature data of Hα emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 M and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find M˙ acc in the 1 - 2 M interval to depend quasi-quadratically on 2.4 0.35 0.7 0.4 stellar mass, with M˙ acc M ± , and inversely with stellar age M˙ acc t− ± . Furthermore, ∝ ∗ ∝ ∗ we compare our spectroscopic M˙ acc measurements with solar Z Galactic PMS stars in the same mass range, but, surprisingly find no evidence for a systematic change in M˙ acc with Z. We show that literature accretion-rate studies are influenced by detection limits, and we suggest that M˙ acc may be controlled by factors other than Z , M , and age. ∗ ∗ Subject headings: stars: pre main-sequence, stars:variables: T Tauri, Herbig Ae/Be, accretion disks, stars: formation, stars: fundamental parameters 1. INTRODUCTION (2005) using similar techniques, where the authors noted Present understanding suggests that most stars accrete that these stars also exhibited Hα emission and were lo- mass from a circumstellar disk during their pre-main se- cated in H II regions. These studies showed that the lu- quence (PMS) phase (see Hartmann 2008). The rate minosities of these objects were significantly higher than ˙ those of Galactic PMS stars at solar Z , and with similar of mass accretion (Macc) from the disk to the star is spectral types. de Wit et al. (2003) showed that HAeBe vital to describe the system’s evolution, including the candidates in the Small Magellanic cloud (SMC) at lower potential growth of planets in the disk whilst the star metallicities (Z 0.002 - 0.004) were even brighter than reaches its main sequence configuration. Viscous disk their LMC counterparts.∼ Based on the inferred lumi- ˙ evolution models predict that Macc drops with time as nosities of the Magellanic clouds (MCs) PMS stars, de ˙ 1.5 Macc t − , a prediction which has been substantiated Wit et al. (2005) suggested that the protostellar mass- by empirical∝ ∗ studies (Hartmann et al. 1998). Observa- accretion rates are inversely proportional to metallicity, tions have also found that the mass-accretion rate scales leading to more luminous PMS stars that are visible at ˙ α with stellar mass as Macc M over a mass range from younger ages at lower metallicities. Whether the PMS 0.3 M -3 M , with index α∝ 2∗ (Calvet et al. 2004; Natta M˙ acc are analogously related to metallicity is not clear. et al. 2006; Muzerolle et al.∼ 2005; Sicilia-Aguilar et al. The M˙ acc estimated by de Wit et al. (2005) for their 2006; Manara et al. 2012), though it has been found to be best LMC PMS candidate (ELHC 7) indicated no signif- arXiv:1412.2014v1 [astro-ph.SR] 5 Dec 2014 discrepant at M < 0.8 M (Fang et al. 2009; Barentsen icant difference in comparison to the canonical Galactic ∗ ˙ et al. 2011). Note that the Macc vs. Mass relation is not average. reproduced by the current paradigm of disk evolution, On the contrary, studies based on the Balmer contin- and it is subject to ongoing research (e.g. Hartmann et uum excess (Romaniello et al. 2004) and Hα emission al. 2006). (Panagia et al. 2000) inferred higher M˙ compared to ˙ acc It has also been suggested that Macc is a function of the Galactic average in a LMC region close to SN1987A. metallicity, Z. Seven PMS star candidates discovered Follow-up Hα photometric studies by the same group in in the Large Magellanic Cloud (LMC) at Z 0.006 by ≈ the LMC regions around SN1987A (De Marchi et al. Beaulieu et al. (1996) were the first PMS candidates 2010) and 30 Doradus (De Marchi et al. 2011a) which identified in a low-Z region. Beaulieu et al. (1996) se- were summarized in Spezzi et al. (2012); and in the lected these objects because they exhibited peculiar pho- SMC open clusters NGC 346 (Z 0.002; De Marchi et tometric variability similar to Galactic Herbig HAe/Be al. 2011b) and NGC 602 (Z 0.004;∼ De Marchi et al. stars. The number of low-Z PMS candidates was in- ∼ 2013) reported a metallicity dependence on M˙ acc (see creased in the follow-up works of Lamers et al. (1999); de ˙ 1 ˙ 0.36 Wit et al. (2002); de Wit et al. (2003) and de Wit et al. below) whilst also finding Macc M , and Macc t in the 1-2 M range for the LMC∝ PMS∗ stars. They∝ ∗ em- 2 V. M. Kalari & J. S. Vink ployed a novel method using VI Hα photometry, rather suggesting that it is challenging for accretion to proceed than conventional spectroscopy to derive M˙ acc. We have at significant rates at older ages in low Z environments. applied a similar method in the Galactic regions IC 1396 It also suggests that M˙ acc may be higher at lower Z, but (Barentsen et al. 2011), NGC 2264 (Barentsen et al. only at young ages. 2013) and NGC 6530 (Kalari et al. 2014, in prep.), and It is thus essential to further scrutinize the differences found that the results from the photometric method do in M˙ acc of metal-poor PMS stars in comparison to solar not differ significantly from spectroscopic Hα or U-band Z PMS stars, as to understand star formation in different measurements, nor did we find a large number of inter- Z environments, as well as the processes involved in disk lopers. Furthermore, we discovered an accreting PMS evolution. To this purpose, we employ spectroscopic ob- candidate star in the LMC region 30 Doradus (Kalari servations of candidate PMS stars located in the Galactic et al. 2014), which is within 10 pc to one of the re- star-forming region Sh 2-284 (Sharpless 1959). Sh 2-284 gions examined in Spezzi et al.∼ (2012). These vari- is an H II region encompassing the central open cluster ous results suggest there may well be genuine highly- Dolidze 25 (also OCL-537). The B stars of Dolidze 25 accreting PMS stars present in the MCs. De Marchi et have been shown to be metal deficient with respect to al. (2013) suggest accretion rates are inversely propor- solar values, with all measured elemental abundances tional to metallicity based on studies of the two SMC ranging from 0.6 to 0.9 dex, leading to an approxi- open clusters. De Marchi et al. (2011b) reported a mate metal deficiency− of− 0.7 dex (Lennon et al. 1990; 8 1 − median M˙ acc of 4 10− M yr− in NGC 346 for a bi- Fitzsimmons et al. 1992). The association of Dolidze 25 modal age distribution× of 1 and 20 Myr; and De Marchi with the Sh 2-284 indicates that the Sh 2-284 region is 8 1 et al. (2013) found a median of 2.5 10− M yr− for also characterized by low metallicity (Puga et al. 2009; × 9 1 Delgado et al. 2010; Cusano et al. 2011). a 2 Myr population in NGC 602 and 4 10− M yr− for a 20 Myr population. De Marchi et× al. (2013) sum- Sh 2-284 is located at a distance of approximately 4 kpc marized their work in the MCs using an equation of the (Delgado et al. 2010; Cusano et al 2010) and provides a unique opportunity to observe PMS stars in the 1 - form log Macc = a log t + b log M + c, where a, b and c are constants, and× c is∗ inversely× proportional∗ to Z. Sim- 2 M range with medium-resolution spectroscopy, and we can compare our measured accretion rates with liter- ilarly, Spezzi et al. (2012) suggest that M˙ acc is inversely ˙ ature spectroscopic measurements of solar Z PMS stars, proportional to Z, and suggest that Macc in the LMC is as well as results claimed from Hα photometry in the higher than that of Galactic PMS stars having similar LMC by Spezzi et al. (2012) 1. masses, irrespective of age. Spezzi et al. (2012) found The paper is organized as follows. Section 2 details ˙ 8 1 a median Macc 5 10− M yr− of LMC PMS stars the data sample, and measured properties of the PMS having an average∼ age× of 10 Myr, when accretion in most stars. Section 3 describes our results and method to cal- low-mass (< 0.8 M ) Galactic PMS stars is thought to culate M˙ acc. A discussion of the M˙ acc distribution with have ceased (Fedele et al.
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